The present invention relates to Hall effect thrusters and, more particularly, to a system for providing the gas with a uniform distribution to a discharge region of the Hall effect thruster.
Ion accelerators with closed electron drift, also known as xe2x80x9cHall effect thrustersxe2x80x9d (HETs), have been used as a source of directed ions for plasma assisted manufacturing and for spacecraft propulsion. Representative space applications are: (1) orbit changes of spacecraft from one altitude or inclination to another; (2) atmospheric drag compensation; and (3) xe2x80x9cstationkeepingxe2x80x9d where propulsion is used to counteract the natural drift of orbital position due to effects such as solar wind and the passage of the moon. HETs generate thrust by supplying a propellant gas to an annular gas discharge region. Such region has a closed end which includes an anode and an open or exit end through which the gas is discharged. The propellant gas is typically introduced into the annular gas discharge region in the vicinity of the anode and, in some systems, through the anode itself. Free electrons are introduced from a cathode into the vicinity of the exit end of the annular gas discharge region. In accordance with the Hall effect, the electrons drift circumferentially in the annular discharge region by a generally radially extending magnetic field in combination with a longitudinal electric field. The electrons collide with the propellant gas atoms, creating ions. Because the ions are generally orders of magnitude larger in mass that electrons, the motion of the ions is not significantly affected by the magnetic field. As a result, the longitudinal electric field accelerates the ions outward through the exit end of the annular gas discharge region, generating thereby a reaction force to propel the spacecraft.
One of the parameters that affects the performance of an HET is the uniformity of the gas propellant as it is introduced into the annular gas discharge region. Researchers believe that when the neutral propellant gas (i.e., before ionization) is concentrated in regions near the anode, electron mobility toward the anode is enhanced. This effect results in locally increased electron current to the anode, which undesirably increases power dissipation and heating of the anode. Nonuniform azimuthal gas distribution in the annular discharge region tends to cause nonuniform azimuthal electron density. It can be shown that the nonuniform azimuthal electron density causes a reduction of the Hall parameter xcex2, which is generally undesirable in HET applications. The Hall effect and the Hall parameter are well known in the art of HETs.
In some conventional HETs, baffles are used to increase uniformity of the gas as the gas is introduced into the gas discharge region. These baffle systems increase gas distribution uniformity to some degree but, of course, greater uniformity is generally desirable. In addition, in some conventional baffle systems, the axial length of the gas discharge region must be made long enough to allow for uniform distribution of the gas after leaving the baffle system. However, the increased axial length of the gas discharge region tends to make the HET susceptible to problems caused by the extreme vibrations and accelerations encountered during launch of the spacecraft into orbit. To avoid these problems, these systems generally increase the thickness and strength of the HET structures to withstand the vibrations. This solution tends to undesirably increase the cost and weight of the HET.
Other conventional systems may use gas injectors to increase gas distribution uniformity. The gas injectors have a large number of injector holes that are uniformly spaced and manufactured to exacting tolerances to achieve high uniformity. However, such injectors are relatively difficult and costly to manufacture. Accordingly, there is a need for a low cost propellant gas distribution system that provides high gas distribution uniformity while being low in size and weight.
In accordance with the present invention, a system for uniformly distributing propellant gas in a HET is provided. In one embodiment, the system is part of an anode assembly that includes an anode and a gas distributor. Propellant gas is directed from a supply to the anode assembly for distribution into the gas discharge region of the HET. In one aspect of the present invention, the gas distributor includes a porous metal xe2x80x9cnozzlexe2x80x9d with an input surface and an output surface. The input surface of the nozzle receives the propellant gas from the supply. Due to the difference in pressure of the propellant gas at the input and output surfaces of the porous metal nozzle, the propellant gas flows through the porous metal nozzle and out of the exit surface into the annular gas discharge region. The porous metal nozzle has an average pore size and thickness that is optimized to control the flow of the propellant gas from the input surface to the output surface at the desired flow rate, pressure drop, and distribution uniformity. Unlike the aforementioned conventional baffle systems which typically achieve gas distribution uniformity at a significant distance from the baffle exit, the porous metal achieves highly uniform gas output flow virtually directly from the exit surface of the gas distributor. This feature allows gas discharge region to be shorter in length compared to conventional systems, allowing the HET to be a low profile compact device that is less susceptible to vibration problems encountered during vehicle launch. Moreover, the porous metal is manufactured to have the desired average pore size, pore distribution and thickness at a cost that is significantly less than the cost to manufacture the previously described conventional injector system.
In a further aspect of the present invention, the gas distributor includes a shield and/or baffle for preventing contaminants from adhering to all or most of the exit surface of the porous metal nozzle. In one embodiment, a shield is implemented with non-porous material and is positioned in the gas discharge region downstream from the anode assembly. In this way, contaminants directed upstream toward the anode assembly are blocked by the shield. Without the shield, the contaminants may clog the pores of the porous metal gas distributor, which may decrease the uniformity of the propellant gas flow into the gas discharge region. The shield interrupts the uniformity of the propellant gas flow and must be positioned far enough upstream of the ion creation zone to diffuse the propellant gas into uniform density again. In a further refinement, the shield may have circular or elongated perforations so as to allow propellant gas to pass through the anti-clogging structure to further decrease the distance needed to achieve uniform gas distribution. The perforations are larger than the pore size of the porous metal gas distributor so that the contaminants do not easily clog the perforations. Although it may be possible for contaminants to flow through the perforations and clog small areas of the porous metal gas distributor, the small areas of clogged pores do not significantly affect the uniform gas distribution provided by the porous metal.
In an alternative embodiment, the anti-clogging structure may be implemented by coating a surface of the porous metal gas distributor that faces generally downstream into the gas discharge chamber. This coating is non-porous and is configured to leave uncovered a surface of the porous metal gas distributor that does not face downstream into the gas discharge region (e.g., the uncovered surface faces in a direction perpendicular to the gas discharge region). That is, the exit surface of the nozzle faces in a radial direction relative to the net gas flow into the gas discharge region. Thus, the probability of contaminants directed upstream from the gas discharge region toward the anode assembly adhering to the uncovered surface of the porous metal gas distributor is significantly reduced.